Key considerations for energy storage system size selection

Interest in residential energy storage is soaring all over Australia – we here at Solar Choice have witnessed a 30% increase in storage-related web traffic since the beginning of the year, and about one third of our new customers express an interest in storage to us. But how much energy storage does a given home require? The answer to this question depends not only on how much electricity the home uses, but also on the degree of energy independence they hope to achieve. This article provides a framework for how to think about energy storage system sizing.

The sliding scale of energy independence

Many Australian homes want to ‘go off the grid’, but in reality this is not an economically viable option for most homes, except where the customer is in an ‘end of grid’ location or in an otherwise remote area. Fortunately, energy independence does not have to only be about going off-grid; a home can choose to have a larger degree of independence without necessarily cutting all ties with their utility.

The list below lays out the various degrees of energy independence (from highest to lowest) that an energy storage system can deliver to a household, depending on budget and ambitions.

  • 100% off-grid: Enough energy generation capacity (solar, small wind and/or diesel generator) and battery storage to allow total energy self-reliance, with no grid connection at all.
  • Mostly self-reliant: Enough energy generation capacity and energy storage to get the household through most days without drawing electricity from the grid, although a grid connection still remains.
  • Summertime self-reliance: Enough solar and battery storage capacity for the home to be energy independent during the typical summer’s day. Power may have to be drawn from the grid in the short days of winter.
  • Peak time self-reliance: Enough solar PV and energy storage to meet household demand during the peak hours of 4pm and 10pm, when electricity is expensive on a time-of-use (TOU) retail tariff. This option would require a smaller investment that the prior three options, but could potentially offer the best returns.
  • Partial peak time self-reliance: If budgetary concerns mean that none of the above options are a possibility, then the household might simply opt to get whatever storage system they can afford, with the aim of reducing the amount of power they need to draw from the grid during expensive peak times.
  • Energy storage for back-up power: A small energy storage system whose main functions are to provide a back-up in the event of a power outage, and possibly also ‘smooth’ the solar energy production curve.

Doing the math: Other considerations

There are significantly more variables involved in selecting an appropriately-sized energy storage system than there are in choosing the right solar PV system size, but once you understand what they are you will be in a better place to make an informed decision. The most important of these factors are:

  • Desired degree of energy independence (as discussed above).
  • Electricity consumption level: If you’re daily electricity consumption is relatively low (less than approximately 20kWh per day), meeting your energy independence goals will be easier and more affordable. If your usage is higher, so will be the amount of investment required to do so. As a first stop, you should look at ways to make your home more energy efficient before selecting a solar-plus-storage system.
  • Electricity consumption habits: According to the analysts at Sunwiz, there are five basic patterns of electricity consumption in Australian homes: 1) ‘Double Peak’, 2) Evening Peak, 3) High Day & Evenings, 4) Day Focus and 5) Night Focus. Examples of these patterns can be seen in the graph below; you can also read more about them in our article about getting the most out of your solar power system.

Common electricity usage patterns

Electricity consumption patterns

The most common energy usage patterns in Australia: Double Peak (blue), Evening Peak (red), High Day & Evenings (yellow), Day Focus (green) and Night Focus (purple).

  • Solar PV system size: If the home in question already has a solar PV system installed, this will be a major consideration in determining how much battery capacity to add: If the home is not a recipient of a state-mandated solar feed-in tariff and the system is exporting excess solar power into the grid, it may make sense to install enough battery capacity to absorb the excess solar power. If the home does have access to a generous feed-in tariff, it might be in their interest to forego energy storage in favour of continuing to receive that ongoing benefit. In cases where both solar array and energy storage are to be installed new, then the household can choose a system that both fits the highest degree of energy independence that their budget will permit.
  • The orientation of your roof/solar panels: North-facing solar panelswill generate more power than east or west-facing panels, but a west-facing array will generally be better suited for meeting afternoon peak time demand.

Energy storage system sizing: Examples for Sydney

Solar Choice is in the process of developing a tool that will help our customers to estimate what size energy storage system will be the best fit for them. This tool will soon be available to the public, but for now we have published some examples from Sydney below. Please note that these results are rough estimates, and should not act as a substitute for recommendations based on your individual circumstances.

Assumptions:

  • Batteries are charged with excess solar – charging batteries using cheap, off-peak grid electricity is not possible.
  • The stored amount of energy available at the beginning of a given day depends on the battery’s remaining state of charge from the night before; there will only be battery power available if there was excess power from the day before.
  • The battery bank’s efficiency in storing and discharging electricity is 95%.
  • Maximum battery capacity is equivalent to recommended depth of discharge (DoD), and the battery is never discharged beyond recommended DoD.
  • If the solar-plus-storage system cannot meet 100% of household electricity demand, use of battery power is restricted to peak & shoulder times (4pm-10pm).
  • Electricity consumption level and pattern do not change between months and seasons (admittedly a big assumption).
  • Solar energy production data from NREL’s PVWatts.

Example 1: Average Sydney home with 5kW system

  • Preexisting north-facing 5kW solar system
  • Average daily household electricity consumption of 25kWh
  • ‘Double Peak’ usage

In this case, the solar energy system is too small to both meet electricity demand and charge the batteries. A battery bank with at least 11kWh of capacity would be required to cover peak & shoulder demand in summer, but about 20% of electricity demand would still need to be met by the grid. You can also see that there is no additional benefit to installing a larger energy storage system in this case, as the solar system is too small to charge it (even though it might be worthwhile if grid charging is possible).

Energy storage system size  Self-sufficient on average day in Dec (summer)?  Self-sufficient on average day in June (winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? % of energy self-sufficiency (on average day)
5kWh N N N N 53%
7kWh N N N N 61%
14kWh N N Y N 77%
20kWh N N Y N 77%

 

Electricity flow profile for Example 1, using 20kWh energy storage capacity

Sydney North 5kW 25kWh Double Peak 20kWh Electricity Flows Overview

Black line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

Example 2: Average Sydney home with 10kW solar system

  • Preexisting north-facing 10kW solar system
  • Average daily household electricity consumption of 25kWh
  • ‘Double Peak’ usage

This example is the same as Example 1, except for a larger solar system – 10kW instead of 5kW. With the larger system it is possible to achieve a higher degree of energy independence with a 14kWh or 20kWh system. Full self-sufficiency is possible even in the wintertime with a 20kWh system, and may be possible with a 14kWh system if the household makes efforts to curb its electricity consumption.

Energy storage system size Self-sufficient on average day in Dec (height of summer)? Self-sufficient on average day in June (height of winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? % of energy self-sufficiency (on average day)
5kWh N N N N 60%
7kWh N N N N 68%
14kWh N N Y Y 94%
20kWh Y Y Y Y 100% (with approx 9kWh spare storage on average day)

 

Electricity flow profile for Example 2, using 20kWh energy storage capacity

Sydney North 10kW 25kWh Double Peak 20kWh Electricity Flows Overview

Black line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

Example 3: Energy-efficient Sydney home aiming for energy independence

  • New-build west-facing 7kW solar system
  • Average daily household electricity consumption of 15kWh
  • ‘High Day & Evening’ usage

Without an energy storage system, a 7kW solar system would send about 70% of the electricity it produces into the grid for this household on the average day. In this case, the goal of sizing the energy storage system would be to absorb the excess solar power for later use (without energy storage the solar capacity should be reduced to 1.5-2kW for optimal financial benefit).

Even though it might seem possible for this household to disconnect from the grid and be fully energy independent, this would not generally be advisable without installing surplus solar and energy storage capacity to cover long periods of bad weather – unless the household is amenable to installing a conventional diesel generator to fill in the gaps.

Energy storage system size Self-sufficient on average day in Dec (height of summer)? Self-sufficient on average day in June (height of winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? % of energy self-sufficiency (on average day)
5kWh N N Y N 77%
7kWh N N Y Y 90%
14kWh Y Y Y Y 100% (with approx 9kWh spare storage on average day)
20kWh Y Y Y Y 100% (with approx 15kWh spare storage on average day)

 

Electricity flow profile for Example 3, using 20kWh energy storage capacity

Sydney West 7kW 15kWh High Day and Evening 20kWh Electricity Flows Overview

Black line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

About Solar Choice

Solar Choice is a residential solar quote comparison service, commercial solar tender manager and utility solar farm developer. Since 2008, our unique software has enabled close to 100,000 households and thousands of businesses to make an informed choice about going solar. On the back of the success of our Solar Quote Comparisons, Solar Choice is currently in the process of developing a quote comparison platform for energy storage providers as well.